Multimode fiber optic links, also commonly known as multimode cables (MMCs) or multimode fibers (MMFs), provide high bandwidth at high speeds. These fibers are referred to as multimode because light can take multiple paths, or modes, through the fiber. A problem with MMFs is modal dispersion caused by differential mode delay (DMD). The effects of DMD become more pronounced with distance. MMFs are thus subject to a relatively constant bandwidth-distance “product.” That is, distance and bandwidth are inversely related, so that as distance increases, the bandwidth decreases. The specifications of a MMF are typically stated in terms of bandwidth-distance at a particular wavelength of light. For example, for a MMF having a 62.5 micron core, the bandwidth-distance specification at 850 nanometers is 160 MHz-km. This fiber supports a data rate of 1.25 Gigabits/second (Gb/s) over 220 meters (m) at a wavelength of 850 nanometers.
A problem arises when it becomes necessary or desirable to increase the data rate beyond that supported by the MMF that has been installed. Of course, one solution is to replace the installed MMF with newer MMF having a higher bandwidth-distance product; however, this is a costly alternative. Other solutions include wavelength-division multiplexing, equalization, and multilevel modulation. Of particular interest to the discussion herein is the use of multilevel optical signals to increase bandwidth. The use of multilevel optical signals is known in the art. In essence, optical signals (or symbols) of different amplitudes are used to encode different sequences of bits at a transmitting node, and the amplitudes are resolved back into bits at a receiving node. For example, in a four-level system, a signal (symbol) will have an amplitude corresponding to one of four levels (including an amplitude of zero). The bits “00” are associated with a signal of zero amplitude, the bits “01” with a signal having a first-level amplitude, the bits “10” with a signal having a second-level amplitude, and the bits “11” with a signal having a third-level amplitude. Thus, in a four-level system, two bits are transmitted per signal (symbol), effectively doubling the bit rate achievable for the same bandwidth.
At the receiving node, the amplitude of an incoming optical signal is compared to threshold values that define the four levels. More specifically, the strength of an electrical signal generated by the incoming optical signal is compared to the threshold values. If the amplitude is less than the first threshold value, the signal is resolved as the bits 00; if the amplitude is greater than the first threshold value but less than the second threshold value, the signal is resolved as the bits 01; and so on. In this manner, a single optical signal (symbol) can be used to transmit multiple bits.
The optical signals are typically generated using some type of laser such as a vertical-cavity surface-emitting laser (VCSEL). A characteristic of these types of lasers is that light output is not always a linear function of electrical current. As current is increased, the amount of light output begins to decrease. Also, the amount of light output is a function of operating time and temperature. Therefore, generally speaking, the amount of light output by a laser can vary. As such, the amplitudes of the optical signals generated by the laser can also vary.
In a multilevel scheme like that described above, it is important to control the amplitudes of the optical signals. Should the amplitude vary too much, a signal may be resolved into an incorrect level, in which case the bits represented by the signal will be incorrectly read. Furthermore, in order to facilitate measurement of signal amplitudes against the thresholds, it is also desirable for the levels in a multilevel scheme to be relatively uniformly spaced. Unfortunately, many of the prior art schemes for controlling signal amplitude cause the distance between levels to become compressed. Consequently, the thresholds may need to be adjusted to keep them within their respective levels; hence, thresholds may be compressed as levels are compressed. The smaller distances between levels (and thresholds) can increase the likelihood that a signal will be resolved incorrectly.
Accordingly, what is needed is a method and/or apparatus that can be used to accurately control the amplitudes of multilevel optical signals used with multimode fiber optic links while maintaining adequate spacing between levels (and between thresholds), especially considering that the light output versus input current characteristics of lasers can change with time and temperature.